1. Field of the Invention
The present invention relates to a dielectric filter and a dielectric duplexer, each said filter and duplexer having a plurality of resonator holes, including at least one resonator hole having two longitudinal portions with different cross-sectional areas arranged in a dielectric block.
The invention also relates to a method of manufacturing the dielectric filter and the dielectric duplexer.
The invention further relates to a radio transceiver utilizing the above-described dielectric duplexer.
2. Related Art of the Invention
A dielectric filter is known in which a plurality of resonator holes are provided in a single dielectric block and a change in the cross-sectional area of each hole is provided by changing the inner diameter of the hole in order to achieve coupling between resonators. In the filter, a line impedance of the resonator corresponding to each resonator hole changes at the boundary where the difference in cross-sectional area is formed.
Such a conventional dielectric filter has, for example, the structure shown in FIG. 14(A) and FIG. 14(B). FIG. 14(A) is a perspective view of the dielectric filter with the surface to be mounted on a circuit board being placed upwards. FIG. 14(B) is a view of the resonator holes viewed from one end of the dielectric block. This dielectric filter is formed of a substantially rectangular-parallelepiped-shaped dielectric block 1 in which two resonator holes 2a and 2b pass through one pair of opposing end surfaces of the block and have inner conductors 3 on their inner surfaces. Input and output electrodes 5 are formed on outer surfaces of the dielectric block 1, and an outer conductor 4 is formed on substantially all of the outer surfaces of the block except for the areas where the input and output electrodes 5 are formed.
Near one end surface 1a' of the dielectric block 1, a gap is formed in each of the inner conductors 3 in the resonator holes 2a and 2b so as to open-circuit (separate) the inner conductors 3 from the outer conductor 4 and so as to generate stray capacitances there. The inner conductors 3 are short-circuited (electrically connected) to the outer conductor 4 at the other end surface 1b of the dielectric block 1 and this end surface 1b will be referred to as a short-circuited end face.
The resonator holes 2a and 2b are provided with steps 21 in substantially halfway along the length thereof so that the inner diameters and cross-sectional areas of the holes change between the open end surface 1a' and the short-circuited end face 1b. Hereinafter, portions having a relatively larger inner diameter in the resonator holes are called large-sectional-area portions, and portions having a relatively smaller inner diameter are called small-sectional-area portions.
Since the large-sectional-area portions are formed at the open-circuited end in the structure shown in FIGS. 14(A)-14(B), a strong capacitive coupling is generally achieved between the two resonators and filter characteristics having a wide pass-band are obtained.
However, in the conventional dielectric filter described above, the fact that the large-sectional-area portion and the small-sectional-area portion of each resonator hole have circular cross-sections, and that their axes are aligned, places limitations on the degree of freedom in the design of the filter. That is, in practice, the resonant frequency of each resonator and the degree of coupling between the resonators are determined by setting the capacitance (hereinafter called self-capacitance) between each inner conductor and the outer conductor, and the capacitance (hereinafter called mutual capacitance) between the adjacent inner conductors. However, only the distance (pitch) between the resonator holes, the length ratio between the large-sectional-area portion and the small-sectional-area portion, and the inner-diameter ratio between the large-sectional-area portion and the small-sectional-area portion can be specified in this design. Thus, when the outside dimensions of the required dielectric block are restricted, it is difficult to obtain filter characteristics over a wide range, since only the above-mentioned measurements and ratios can be adjusted. Conversely, if a dielectric block satisfies the required filter characteristics, its outside dimensions may not fall in a desired range.